Watermarking is a technique in which information is combined with or represented in some way in host material such as video images. There may be various reasons for representing the information in the material. In some applications, the information is combined with the material in a way which makes it as difficult as possible to detect the presence of the information. In other applications the video material may be watermarked with the effect that the information is visible within the video image.
UK Patent application 0029852.1 discloses an arrangement for introducing a visible watermark into a video image. The visible watermark may be introduced by selecting parts of digital samples of the video image, and modifying the samples to represent the visible watermark. Furthermore, the watermark is encoded in the video material in a way, which allows it to be removed to recover the original video images. The watermark is therefore encoded into the video image with a reversible function. Also disclosed in this UK Patent application is that a bitmap template may be formed to represent the samples of the image that are to be changed to represent the watermark.
UK Patent application 0215403.7 discloses an arrangement for introducing a watermark into an MPEG-2 encoded video image. The watermark is introduced by adapting the DCT coefficients of the MPEG-2 video image in a way, which does not increase an amount of the compression encoded data representing the video image. Those parts of the video image which are changed in order to represent the watermark are removed and introduced into an modified MPEG-2 bit stream in spare data areas appended to the end of the bit stream.
UK Patent applications 0029850.5 and 0121997.8 disclose an arrangement for introducing a visible watermark into a compression encoded bit stream by making pseudo random changes to the bit stream in accordance with a substantially invertible algorithm. The changes are invertible with the effect that the changes can be removed, and because the algorithm does not substantially change the number of entropy encoded bits, so that the compression encoded bit stream can be decoded with a conventional decoder. The visible watermark is introduced by changing a Discrete Cosine Transform (DCT) coefficients, which represent the encoded video image.
Some applications, visible watermarking can provide an arrangement for previewing video content, in order to allow the viewer the opportunity to sample the content before committing to acquire that content. Thus, for example, a viewer can preview the content before buying that content, the previewed content including the watermark so that copying or using the content without payment is discouraged. As explained above, since a visible watermark has been added in a way, which is invertible, a viewer may be sent data for removing the watermark upon payment.
Although these known watermarking techniques can be used to form visible watermarks in compression encoded bit streams, the complexity of the compression encoding can make it difficult to adapt the visible watermark within the video images.